1. Field of the Invention
This invention relates in general to applying a constant load against a medium subject to large displacements and, in particular, to the manufacturing of an assembly capable of providing such a constant load. More specifically, but without restriction to the particular embodiment hereinafter described in accordance with the best mode of practice, this invention relates to a method and apparatus for making a bubble spring assembly having a predictable and consistent stress strain history along the length of the spring.
2. Discussion of the Related Art
Bubble spring assemblies have recently been employed in a number of electrical and mechanical devices including tape decks, sound recording devices, and automated materials handling machinery. Bubble spring assemblies are commonly employed in a situation requiring a constant load with the ability to accommodate large spacial displacements. A typical application for the bubble spring assembly is to provide a restraining force against a moving piece of magnetic tape so that the tape is held against a recording head under constant pressure.
A bubble spring assembly typically includes a cavity block and a bubble spring. The cavity block includes a shaped hollow cavity formed through the block. The bubble spring is typically small and thin. Delicate handling of the spring is therefore required. The bubble spring is positioned within the cavity to form a convex or arched portion extending beyond the dimensions of the cavity block. In this manner, the bubble spring is placed under tension and will thereby provide a spring force against an article that has displaced the spring into the cavity. Prior to shaping and insertion into the cavity block, the bubble spring is a flat elongated piece of thin material, typically metal, commonly referred to as a spring blank. This type of bubble spring assembly is disclosed in commonly assigned U.S. Pat. No. 5,274,522 issued to P. L. Taillie, Dec. 28, 1993, entitled "Magnetic Head-To-Media Backer Device". An improved bubble spring assembly of this type is disclosed in commonly assigned, copending U.S. patent application Ser. No. 08/431,708 filed Apr. 28, 1995 by D. W. Ryan et al. similarly entitled "Magnetic Head-To-Media Backer Device". These two references are each directed to a bubble spring assembly device and optimized configurations therefor.
It is now understood that assembly techniques for manufacturing the bubble spring assembly play a crucial role in making consistent assemblies having a predictable stress strain history within a required range of tolerances. Consistency in stress strain history of the bubble spring is required so that a specified and predictable force is provided by the spring. Spring blanks are typically loaded into a respective cavity block by skilled assemblers using either industrial tweezers or finger manipulation. These hand techniques may vary from person to person thus resulting in slightly different stress strain histories in a manufactured lot of bubble spring assemblies. This situation is undesirable since a purchaser of large quantities of bubble spring assemblies may require that all purchased bubble spring assemblies perform within exacting tolerances. An additional drawback associated with these hand assembly techniques is that an assembler may become fatigued or lose concentration and plastically deform a spring blank during loading. In this case, the spring blank is unusable and must be replaced since any resulting spring force is severely weakened. Thus, prior hereto, there has not been proposed a method and device for manufacturing a bubble spring assembly with a consistent and predictable stress strain history maintainable over long production periods.